To find the assassin, follow the road west from Thrallmar. Go past Hellfire Citadel and the Pools of Aggonar. Once you pass the pools, start following the hills on the north side of the road. You'll eventually find the assassin, or what's left of him, at [33, 43].

IMPORTANT: This quest chain must get completed in order to get friendly with the Mag'har. If you don't complete it, you won't be able to talk to most of the people in Garadar in Nagrand. Alternately you can do the Garadar quest The Impotent Leader.

UPDATE: The above is no longer the case in 4.0.1. The last 2 quests in the chain have been removed completely. Thus, after completion of the chain, reputation will be at about 700/3000 Neutral.